Anhydrous ammonia (NH.sub.3), which is 82% nitrogen, is applied to soil by farmers as a fertilizer. Farmers often use a nurse tank containing pressurized liquid NH.sub.3 as the source. This tank is transported by the farm vehicle across a field while the NH.sub.3 is distributed to the soil. An over application of NH.sub.3 costs the farmer money, and an under application affects the crop. Moreover, since groundwater contamination attributable to NH.sub.3 has become a more prominent issue (now regulated by some states), it is desirable to accurately control the flow of NH.sub.3.
The crudest method of controlling the flow of NH.sub.3 to the soil is to partially open a ball valve and roughly calculate the flow rate of NH.sub.3 to the soil. This may be done by reading the percentage the tank is full with a meter on the tank. The farmer then makes a test run and, based upon speed, acreage and the amount of NH.sub.3 used, he calculates flow. Several test runs followed by valve adjustment may be necessary to achieve the desired flow rate. If the tank pressure gauge indicates a change in pressure during the course of a day as a result of the NH.sub.3 warming to the daily outdoor temperature, the flow rate has changed even though the valve position remains fixed. Accordingly, another test run may be needed. Needless to say, this method is crude and burdensome.
More accurate flow measurement for real time flow control of NH.sub.3 presents a rather difficult problem. NH.sub.3 has a low boiling point (low vaporization temperature). Pressure drops result in flashes (liquid turning to vapor) in the NH.sub.3, and the created vapor makes the flow measurement inaccurate. Without an accurate measurement of the flow rate, the farmer cannot properly control the application of NH.sub.3. A number of variables can cause the flow rate to change, including the ground speed of the farmer's vehicle, the temperature within the nurse tank and flow lines, soil density, the desired application, and the flow position of the regulator or valve (ranging from fully closed to fully open). Moreover, the farmer has no control over soil density or the temperature within the nurse tank, which can vary greatly during the course of a day. The prior art teaches that accurate measurement of the NH.sub.3 flow rate requires condensation after the NH.sub.3 is two phased (liquid/vapor). Heat exchanger and/or NH.sub.3 liquifiers for performing this condensation purpose are expensive and require high maintenance.
Although others recognized the problem the farmer experienced in controlling the amount of NH.sub.3 to be applied to the soil, the prior art has failed to devise a simple, accurate and inexpensive system for resolving the problem. For over 20 years, prior art systems attempted to obtain a more accurate measurement of the flow rate by taking the two phase NH.sub.3 returning it to a single liquid phase, and then measuring the flow rate of this liquid. A continuing problem with such systems are their expense. Moreover, the condenser incorporated into the system never fully converts the two phase NH.sub.3 back to liquid, and the condenser inherently uses a restriction in the flow path. The severity of the restriction increases during cold weather or low pressures. The cost of these systems for a typical farmer is commonly prohibitive, or is unjustifiable given the savings to be realized. The condensers are commonly designed for a specified flow rate, and at flow rates exceeding the specified flow rate, the condenser has difficulty converting the vapor to liquid, thereby reducing the accuracy of the flow measurement. In teaching that more accurate flow measurement required the taking of two phase NH.sub.3 and returning to a single phase with a heat exchanger, the prior art devices taught away from the present invention.
In most prior art NH.sub.3 dispensing systems, the flow meter is the most delicate component. Numerous types of flow meters have been devised, including both variable cross-sectional area flow meters wherein the cross-sectional flow cavity through the flow meter is indicative of flow rate, and turbine flow meters wherein the angular velocity of the turbine is proportional to the flow rate. With regard first to variable area flow meters, it is known that such flow meters may be devised such that the flow rate is related to the position of a member which defines the cross-sectional flow area through the meter at any point in time. Prior art variable area meters have several significant drawbacks, however, which have resulted in these meters not being acceptable for use in measuring the flow rate of anhydrous ammonia. Some of these meters include a sensor mounted on the vane shaft, but a seal is required between the flow chamber and the sensor. One patent disclosing such a meter is U.S. Pat. No. 3,835,373. The seal is subject to a highly hostile environment when the meter is used for fluids such as anhydrous ammonia, and accordingly this type of meter would not generally be considered acceptable for use on an anhydrous ammonia distribution system.
Another type of variable area meter utilizes a magnet mounted on a vertically suspended body and a hall sensor to provide an electronic output of the position of the suspended body and thus the flow rate through the meter. A system of this type is disclosed in U.S. Pat. No. 5,187,988. This meter would typically not be suitable for use in the application discussed above since the meter must be positioned in a true vertical position for proper flow measurement. Many fields commonly have rolling hills, and both the tractor and the equipment pulled by the tractor are thus not always moving truly horizontally. The flow meter discussed in this patent and the vertically suspended body in particular would also be highly susceptible to inaccurate readings and/or damage if subjected to vibration of the type common to farming equipment. This meter is also designed for a very low pressure application, and anhydrous ammonia is typically dispensed at medium or high pressures in excess of 250 psi.
Other variable flow area type devices are disclosed in U.S. Pat. Nos. 5,497,081, 5,444,533 and 5,327,789. Many types of these flow measurement devices are frequently designed to operate in the vertical position. Complicated sensor assemblies are frequently employed to detect the position of the flow area defining member. These complicated detector and sensor assemblies are very costly, and are not highly reliable when used in the rugged environment required for farming equipment. Other variable area meters employ complicated flux concentrators. Mechanical calibration or remote read-out devices which are generally unsuitable for anhydrous ammonia applications are also commonly associated with variable area flow meters, as disclosed in U.S. Pat. No. 4,487,007. Farmers want a meter which has a low cost and which is not complicated or difficult to calibrate. As explained above, many prior art variable area flow meters must be positioned vertically to be accurate, and this restriction is unacceptable to NH.sub.3 applications. U.S. Pat. No. 5,444,369 discloses another type of variable area meter. Various pole pieces must be precisely positioned in order to provide a desired linear output between the flow and the electronic output from the hall device. Prior art meters which rely upon a variable area concept for measuring flow have thus long been considered too expensive, too complicated, too delicate, and too limiting for anhydrous ammonia use.
Almost all flow meters currently in commercial use for measuring the flow rate of anhydrous ammonia applied from the nurse tank to the field are of a type which employ a rotating turbine, wherein the rotational output of the shaft is proportional to the flow rate. These turbine-type meters common employ a magnetic pick off on the shaft, so that each rotation of the shaft produces an output signal, the number of pulses or signals generated during any period of time is thus used to determine the flow rate of anhydrous ammonia through the meter. Turbine-type meters are quite expensive, but are generally considered rugged and do not require precise positioning to provide an output. Unfortunately, a significant disadvantage of such meters when used for measuring the flow of fluids which are easily vaporized is that the meters are frequently damaged when the liquid nurse tank runs dry.
The absence of liquid flowing through the turbine meter and the presence of only vapors commonly damages either the meter or the other system components whose operation is affected by the meter output. Problems have thus commonly arisen with respect to the use of the turbine meter in prior art anhydrous ammonia distribution systems. When the NH.sub.3 nurse tank runs empty, high velocity vapor passing through the turbine meter causes the impeller to spin at extremely high speeds. The meter bearings typically quickly fail or develop excessive wear, thereby causing flow reading errors. This cause for failure is present in any system with a turbine meter, even if the system is equipped with a heat exchanger to remove vapor. The heat exchanger requires liquid input to perform its intended operation, and when the nurse tank runs empty, only vapor flows through the heat exchanger and the turbine meter. Although turbine meters are thus widely used to measure the flow rate of anhydrous ammonia being applied by a farmer, these meters have high repair and maintenance costs.
The disadvantages of the prior art are overcome by the present invention, and an improved system for reliably measuring the flow rate of low temperature vaporization fluids, such as anhydrous ammonia, are provided so that flow may be reliably regulated in response thereto. The flow meter of the present invention is particularly well suited for use in measuring the flow rate of anhydrous ammonia which is applied to the field from a portable nurse tank.